Tips for Drawing in Public?

In my advanced drawing class, we are focusing on multi-figural compostions, requiring me to draw in public a lot. I've been having trouble with this, because my teacher wants us to draw at the Sports and Rec Center. It's really hard to draw people doing sports or exercising, because they are constantly moving. I find that I'm only able to get a quick glance at what they're doing, like a 1 second snapshot, and have to furiously scribble down what I can remember. Thus, I've found myself mostly drawing from memory, and many of my drawings are crap. Even a 10 second pose on posemaniacs is easier.

Does anyone have any tips on how to go about this? I know that the obvious answer is to practice practice practice. But apart from that, does anyone have any practical tips on how to do this?

I am sure the drawing is to learn the movements of the body, and how each part changes for muscle movment, hinging of the joints, tendons pulling, and the action of the poses.

The best way to get started is to study some of this stuff happening to the body, then when you see a pose you feel like drawing start with a sketch of the pose, so you will have an idea of the position each part is in. Then for each part you are drawing, wait for it to get in that possition again, so you can see how the muscles look and draw them. You do that for each part, because they are more likely to be contantly repeated. All you really want to do is study the movements, and draw to help you remember how the body looks as it moves. When you make corrections you learn more about the anatomy.